Knowing some classic literature works being free... what would be typical sites to find and download those? (By "classic" I mean those older works that are now regarded to be “copyright-free”.)

I know iTunes, GooglePlay and the rest offer some free literature works via apps, but I am looking for more "open" resources like free online libraries, archives, etc. Does anything alike exist? Is there a directory or something where (copyright-) free books are listed for reference purposes?

The reason why I am asking is that I would hate having to waste time scanning my hardcopies just to read them on my ebook reader, when such books are freely available in ebook-compatible format somewhere...

Note: I'm talking about freely available literature, and not stolen (non-free) works offered by illegal download sites, and certainly not things like useless "get rich quick" PDF guides. I did my homework and (ab)used some search engines, but in contrast to my usual experiences, search engines didn't really help as most of what they suggest turned out to be sites offering illegal downloads.

The only usable example I found and which had an acceptable quality was Archive.org's texts. In fact, it is a pretty good example of what I'm looking/asking for. There must be more, alike resources out there…

Shopping question means: Recommend me a list of things or resources, one complete answer is impossible, and I don’t care about the inevitable link rot. It is not about the money, but the quality of the question. These kind of questions tend to attract spammers, the links will probably not work or lead to malware sites over time.
– fuxiaDec 20 '13 at 11:30